This invention relates to an anti-fouling overcoating composition for watercraft and underwater portions of marine structures precoated with a hard-surface leaching-type anti-fouling paint and to a method for reducing fouling and, secondarily, drag against flowing water of underwater structures, whether movable or stationary.
Marine fouling organisms, such as crustaceans (barnacles, Conchodermae), mollusks (boring worms and boring clams), annelids (tubeworms, Serpulidae), tunicates, algae, coelenterates, acorn shells (Balonidae), goose mussels (Lepodoids), and moss (Hydroidae) grow and accumulate on surfaces in an underwater environment. Wooden structures are particularly susceptible to attack by pileworms or shipworms (Teredinidae), limnoria, martesia, sphaeroma and the like. Metal water intake pipes for cooling towers in thermoelectric power stations, petrochemical factories or the like are commonly infested with and fouled by adherence of Blue mussel, sessile acorn barnacles (Balanus), oysters, moss (Bryozoa), Hydrozoa and the like.
"Fouling" can be defined as assemblages of marine animals and plants which grow on watercraft and underwater marine structures, as well as on rocks, stones and other natural objects. Marine fouling is a result of growth of marine animals and plants on the exposed surfaces of man-made marine structures, including bottoms and hulls of ships, pier supports, buoys, water-intake pipes, fishing nets, rowboats, water skis, ocean liners, tankers and other cargo ships, submarines, pilings, bridge substructures and the like. Marine fouling is an omnipresent problem, occurring whether the underwater surface is made from wood, metal, plastic, fiberglass, concrete or other structural material.
Fouling is economically harmful to the shipping industry because of the accompanying decrease in the speed of watercraft, resulting in higher power consumption and operating costs to maintain schedules, and because of the need to bring ships into drydock to remove the fouling and apply fresh paint. Furthermore, fouling can produce physical damage to watercraft or stationary underwater structures, resulting in shortened useful life and the need for frequent replacement. In the case of fishing nets, fouling causes an increase in the fluid resistance to water flowing therethrough, so that fish growing in a limited area, as is typical for areas of cultivation, are adversely affected by a poor supply of oxygen in the water.
Attempts to control fouling by marine organisms generally have been based on the use of chemicals which are toxic to a specific organism or group of organisms. Thus, creosote has been used for the impregnation of wooden surfaces. However, creosote is unsatisfactory for painted surfaces, for the reason that creosote bleeds through the paint.
A variety of anti-fouling agents have been developed which are toxic to deleterious marine organisms. Typical of these agents are cuprous oxide, mercury oxide or the like; organocopper salts, e.g., copper naphthenate, copper oleate, etc.; organotin compounds, e.g., bis-tributyl tin oxide, triphenyltin bromide, dibutyl ethyltin bromide; 1,2,3-trichloro-4,6-dinitrobenzene; dichlorodiphenyldichloroethane; nitrodiphenyl ether derivatives,; organolead compounds, e.g., triphenyl lead stearate, triphenyl lead chloride; 10,10'-oxybisphenoxazine (SA-6 546); hexachlorophene and tetrachloroisophthalonitrile, alone or in combination. However, most of these materials are highly toxic to other organisms and tend to leach out of the coating in which they are applied so that the coating composition becomes ineffective in preventing fouling after a period of time. Often, compositions containing these materials are difficult to apply to marine structures for lack of adequate adhesion or tend to peel from the structure after an unsatisfactorily short interval.
It is therefore apparent that there is a continuing need for anti-fouling systems which adhere tenaciously to the surface treated therewith, operate efficiently as antifoulants over a prolonged period of time and are not highly toxic to desirable forms of marine life.